The present invention relates to moving target indication (MTI) radars, in general, and more particularly to a radar channel including two different orthogonal MTI filters operating functionally in parallel with substantially similar target signal response characteristics, except that their noise outputs are uncorrelated for a post detection integration thereof.
An MTI radar channel is exemplified in the block diagram schematic embodiment depicted in FIG. 1. Typically, received echo signalling of the radar is conditioned into intermediate frequency (IF) or video signalling 10 and provided to a conventional synchronous detector 12 wherein it is separated into its in-phase (I) and quadrature (Q) component signals. In most modern radars, the I and Q component signals are digitized by a sampling analog-to-digital (A/D) converter 14 to provide sampled-data I' and Q' component signals which are temporarily stored in a memory 16. The memory 16 operates in conjunction with an MTI filter 18, which may be an N-pulse canceler, for example, to process signals I'.sub.i and Q'.sub.i in accordance with a common range cell i.
The primary purpose of the MTI filter 18 is to effect substantial notch rejection of doppler frequencies about zero frequency, the pulse repetition frequency (PRF), and harmonics thereof. The desired notch rejection characteristics of the filter 18 generally are a function of the number of radar pulses the filter is operating on concurrently. Practically, the MTI filter 18 may be designed for two-, three-, or four-pulse canceller operation under which conditions, the filter 18 is operating concurrently on two-pulses, three-pulses, or four-pulses, respectively, of the stream of echo pulses received by the radar. Thus, once receiving its requisite number of pulses under an initialization phase, the filter 18 operating in conjunction with the memory 16 generates real X.sub.i and imaginary Y.sub.i pulsed signalling for each new echo pulse received and conditioned by the radar. The signals X.sub.i and Y.sub.i are coherent doppler frequency signals within the pass band of the filter 18 resulting from a common range cell i.
A conventional amplitude detector 20 may be included in the radar channel to derive the amplitudes of the coherent signalling X.sub.i and Y.sub.i and provide this sequence of amplitudes via signal line 22 to a post detection integration (PDI) circuit 24 for the non-coherent integration thereof. Generally, the integration process of the circuit 24 may be conducted in accordance with a well-known "sliding-window" technique. The resulting stream of integrated pulsed signalling may be compared with a threshold level in a conventional threshold circuit 26 for the purposes of target identification. Ordinarily, the pulsed signalling exceeding the threshold level is an indication of a target and not extraneous noise. In effect then, the MTI filter 18 is provided to eliminate clutter such as surrounding terrain, rain, chaff, slow-moving clouds and the like; and wherein the PDI circuit 24 is provided to improve the signal-to-noise (S/N) echo signalling passed by the MTI filter 18 and thus provide for better sensitivity in the threshold circuit 26 for detecting actual targets.
Detectability in a typical radar MTI channel, which may be averaged for example over all target radial speeds as represented by the measured doppler frequency thereof, is generally lower than that in a corresponding non-MTI or normal channel, even when there is no significant clutter interference present. This is because the MTI filter 18 produces correlation in the thermal noise component of its outputs X.sub.i and Y.sub.i, resulting in a lower pulse detection integration grain. For example, when there is no target present, only noise is present in the interpulse echo signalling pulse-to-pulse which limits sensitivity inherently. That is, a large pulse noise signal will be processed by the N-pulse canceller 18 for the interval of N-pulses due to the correlative operations of the MTIfilter, i.e. a piece of noise data input to the filter will stay in the filter for the number of pulses corresponding to the order of the canceller and thus contribute to the sum output over the length of time associated therewith, as weighted by the different tap weight factors at each node of the filter it passes. The effect of this operation is to make the noise contribution of the output signals X.sub.i and Y.sub.i correlated from pulse-to-pulse for the number of pulses corresponding to the order of the canceller. In contrast, in non-MTI channels, the noise signalling is statistically independent from pulse-to-pulse rendering no correlation thereof.
The threshold level of the threshold circuit 26 is normally selected to maintain a constant false alarm rate in the absence of a target echo signal. In determining the false alarm characteristics of the threshold circuit 26, one must take into account whether or not a correlation process is present in the radar channel. When a heavy correlation is present in the MTI filter 18, for example, for a given false alarm behavior of the radar, the threshold level of the threshold circuit 26 is set higher than in the case in which there is no noise correlation. With a higher setting of the threshold level, the threshold circuit 26 is less able to detect a target echo signal because the signal has to be stronger to cross the higher threshold than in an uncorrelated noise process. For this reason, there is loss in detectability associated with the MTI filter basically because the threshold level in the threshold circuit 26 is raised in order to maintain the desired constant false alarm rate which makes the radar less sensitive with regard to target detection.
The MTI radar channel loss is particularly severe in a frequency diversity radar operating in a pulse burst MTI mode. Such a radar may transmit N pulses, where N may be on the order of four, for example, at a single carrier frequency and process the return echo signalling through an N-pulse MTI canceller such as that shown in the radar channel of FIG. 1, thus producing only a single output pulse for detection in the threshold circuit 26 downstream, prior to changing the carrier frequency and repeating the burst process. In this case, no PDI is possible in the MTI channel, whereas, in a corresponding normal channel, PDI of N-pulses can be accomplished. Thus, there is even a more significant loss penalty for the frequency diversity operation, i.e. one pulse output for each N-pulse burst, wherein the PDI 24 can no longer perform its sliding window, non-coherent integration.
It has been proposed to obtain two pulses out of an N-pulse canceller filter with an N+1 pulse burst and integrate the two pulses non-coherently in an attempt to get approximately 3 db (i.e. 2:1) improvement over the present loss in detectability. In reality, this proposal does not afford such an improvement because the noise from pulse-to-pulse is heavily correlated in the processing of the N-pulse canceller filter in which case it does not do any good to add up the two successive pulses generated therefrom. For example, for a burst of five pulses in a frequency diversity operation, a four-pulse canceller generates two successive pulses for each burst of different frequency. However, the integration of the two successive pulses doesn't provide any significant detection performance benefits because of the noise correlation of the MTI channel processing.
What apparently is needed especially with regard to the frequency diversity operational mode of the radar is an MTI radar channel structure and operation which provides uncorrelated noise outputs which may be combined non-coherently in order to provide significant detection performance benefits without alternating substantially the notch rejection frequency response of the MTI filter characteristics.